This invention relates to a data processing system, and more particularly, to an apparatus for providing safe-state outputs in a failure mode of a process control system.
In a process control system, a process is controlled which generally involves the monitoring and controlling of fluid flow, fluid flow rate, fluid levels, temperature, pressure, position, levels (e.g. tank or vat levels), . . . These parameters are resultants of a process and are measured by a variety of field devices including such devices as valves, flow meters, position indicators, motors, . . . As the process continues, the parameters are being monitored by a main controller, and output signals are generated for controlling the various field devices (or field equipment) or interposing control relays that connect to the field devices. An apparatus, referred to herein as an output device, interfaces with the field devices and outputs control signals (or command signals) to the field devices. In the event of a failure in the system, there is a need to output signals to the field devices which define a predetermined, selectable (i.e., configurable) default condition for the devices. The apparatus of the present invention provides a predetermined output signal to the field devices, including a way of "online" testing the apparatus.